<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<style>
#div1 {width:100px; height:100px; background:red; position:absolute;}
#div2 {width:700px; height:500px; background:#CCC; position:relative;}
</style>
<script>
window.onload=function ()
{
	var oDiv=document.getElementById('div1');
	var oDiv2=document.getElementById('div2');
	
	var disX=0;
	var disY=0;
	
	oDiv.onmousedown=function (ev)
	{
		var oEvent=ev||event;
		
		disX=oEvent.clientX-oDiv.offsetLeft;
		disY=oEvent.clientY-oDiv.offsetTop;
		
		document.onmousemove=function (ev)
		{
			var oEvent=ev||event;
			var l=oEvent.clientX-disX;
			var t=oEvent.clientY-disY;
			
			if(l<50)
			{
				l=0;
			}
			else if(l>oDiv2.offsetWidth-oDiv.offsetWidth)
			{
				l=oDiv2.offsetWidth-oDiv.offsetWidth;
			}
			
			if(t<50)
			{
				t=0;
			}
			else if(t>oDiv2.offsetHeight-oDiv.offsetHeight)
			{
				t=oDiv2.offsetHeight-oDiv.offsetHeight;
			}
			
			oDiv.style.left=l+'px';
			oDiv.style.top=t+'px';
		};
		
		document.onmouseup=function ()
		{
			document.onmousemove=null;
			document.onmouseup=null;
		};
		
		return false;
	};
};
</script>
</head>

<body>
<div id="div2">
	<div id="div1"></div>
</div>
</body>
</html>
